Amplifiers and voltage shifters are extensively used in integrated circuits. There are various types of amplifiers such as feedback amplifiers, differential amplifiers and operational amplifiers. They can be used in circuits that handle either digital signals, analog signals and mixed signals. For example, voltage shifters and amplifiers are used to shift the voltage level or amplify an input signal to be send to a comparator, enabling a more accurate comparison between the amplified/shifted input signal and another signal provided to the comparator. U.S. Pat. No. 4,471,244 of Maples, U.S. Pat. No. 5,087,834 of Tsay, U.S. Pat. No. 5436866 of Jang and U.S. Pat. No. 5,889,389 of Bothra demonstrate a small portion of the various uses of voltage shifters and amplifiers.
Many electrical devices have integrated circuits that handle mixed signals. A cellular phone is one of these devices. Cellular phone often have a sigma delta modulator, which are used to convert signals between analog and digital domains.
Sigma delta modulators comprise of a noise shaping element, a comparator and a feedback loop. Usually, an output of the noise shaping element is coupled to an input of the comparator, and an output of the comparator is coupled via a feedback loop to an input of the noise shaping element. When the noise shaping element provides a single output signal, the comparator compares between the single output signal and a reference signal. When the noise shaping element provides two differential output signals, the comparator compares one differential output signal to the other.
The noise shaping element can have either a discrete or a continuous transfer function. A common noise shaping element is an integrator. A sigma delta modulator having an integrator receives an analog signal, provides the analog signal to an integrator for integrating the analog signal, sends an output signal of the integrator to a comparator for comparing the output signal to a reference signal, and produces a digital output signal which is related to the analog signal, the digital output signal is also provided to a feedback circuit within the sigma delta modulator. A premium is placed upon the speed and accuracy of the sigma delta modulators, which are partly limited by the comparator accuracy. On one hand there is a need to perform an accurate comparison, and not to output erroneous results and on the other hand accurate measurements may be time consuming, thus limiting the speed of the sigma delta modulator.
A premium is placed upon low energy consumption, especially in electrical devices such as cellular phones powered by batteries. Most prior art amplifiers/voltage shifters were relatively energy consuming, and were comprised of a relative large number of components. Furthermore, such amplifiers and voltage shifters consumed valuable die area. A premium is also placed upon components that can work at a very high frequency, especially in the network and the telecommunication fields.
A premium is also placed upon accuracy and the ability of a device to work in relatively noisy environments. For example, the comparator within the sigma delta modulator has to determine whether the analog input signal is above or below a reference signal. If the analog input signal is very close to the threshold, and especially when an unwanted noise is added to the analog input signal, the comparator can produce an erroneous result. Usually, the accuracy of the comparator can be improved by amplifying the analog signal, but the amplification involves larger energy consumption, and most of the amplifiers tend to add an unwanted offset signal to the analog signal.
There is a need of a compact-sized, high-speed and low energy consuming voltage shifter/amplifier. There is a need for a high speed and accurate sigma delta modulator that has a compact size- and low-energy consuming voltage shifter/amplifier.